dual-tone multi-frequency signaling (dtmf

A DTMF keypad from anAutovon Telephone. The

column of red keys producesthe A, B, C, and D DTMFevents, labeled by their

telephony function.

Dual-tone multi-frequency signalingFrom Wikipedia, the free encyclopedia

Dual-tone multi-frequency signaling (DTMF)is used for telecommunication signaling overanalog telephone lines in the voice-frequencyband between telephone handsets and othercommunications devices and the switching center.The version of DTMF that is used in push-buttontelephones for tone dialing is known asTouch-Tone, was first used by AT&T in commerceas a registered trademark, and is standardized byITU-T Recommendation Q.23. It is also known inthe UK as MF4.

Other multi-frequency systems are used forinternal signaling within the telephone network.

The Touch-Tone system, using the telephonekeypad, gradually replaced the use of rotary dial starting in 1963, and sincethen DTMF or Touch-Tone became the industry standard for both cell phonesand landline service.[1]

Contents

1 Multifrequency signaling2 #, *, A, B, C, and D3 Keypad4 Special tone frequencies5 See also6 References7 Further reading8 External links

Multifrequency signaling

Prior to the development of DTMF, automated telephone systems employed pulsedialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling to dialnumbers. It functions by rapidly disconnecting and re-connecting the calling

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party's telephone line, similar to flicking a light switch on and off. The repeatedinterruptions of the line, as the dial spins, sounds like a series of clicks. Theexchange equipment interprets these dial pulses to determine the dialednumber. Loop disconnect range was restricted by telegraphic distortion andother technical problems , and placing calls over longer distances requiredeither operator assistance (operators used an earlier kind of multi-frequencydial) or the provision of subscriber trunk dialing equipment.

Multi-frequency signaling (see also MF) is a group of signaling methods, thatuse a mixture of two pure tone (pure sine wave) sounds. Various MF signalingprotocols were devised by the Bell System and CCITT. The earliest of these werefor in-band signaling between switching centers, where long-distance telephoneoperators used a 16-digit keypad to input the next portion of the destinationtelephone number in order to contact the next downstream long-distancetelephone operator. This semi-automated signaling and switching provedsuccessful in both speed and cost effectiveness. Based on this prior success withusing MF by specialists to establish long-distance telephone calls, Dual-tonemulti-frequency (DTMF) signaling was developed for the consumer to signaltheir own telephone-call's destination telephone number instead of talking to atelephone operator.

AT&Ts Compatibility Bulletin No. 105 described the product as "a method forpushbutton signaling from customer stations using the voice transmission path."In order to prevent using a consumer telephone to interfere with the MF-basedrouting and switching between telephone switching centers, DTMF's frequenciesdiffer from all of the pre-existing MF signaling protocols between switchingcenters: MF/R1, R2, CCS4, CCS5, and others that were later replaced by SS7digital signaling. DTMF, as used in push-button telephone tone dialing, wasknown throughout the Bell System by the trademark Touch-Tone. This term wasfirst used by AT&T in commerce on July 5, 1960 and then was introduced to thepublic on November 18, 1963, when the first push-button telephone was madeavailable to the public. It was AT&T's registered trademark from September 4,1962 to March 13, 1984,[2] and is standardized by ITU-T Recommendation Q.23.It is also known in the UK as MF4.

Other vendors of compatible telephone equipment called the Touch-Tone featureTone dialing or DTMF, or used their own registered trade names such as theDigitone of Northern Electric, now known as Nortel Networks.

The DTMF system uses eight different frequency signals transmitted in pairs torepresent sixteen different numbers, symbols and letters - as detailed below.

As a method of in-band signaling, DTMF tones were also used by cable televisionbroadcasters to indicate the start and stop times of local commercial insertionpoints during station breaks for the benefit of cable companies. Until betterout-of-band signaling equipment was developed in the 1990s, fast,


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DTMF keypad layout.

unacknowledged, and loud DTMF tone sequences could be heard during thecommercial breaks of cable channels in the United States and elsewhere.[citation needed]

#, *, A, B, C, and D

The engineers had envisioned phones being usedto access computers, and surveyed a number ofcompanies to see what they would need for thisrole. This led to the addition of the number sign(#, sometimes called 'octothorpe' or 'pound' in thiscontext - 'hash' or 'gate' in the UK) and asterisk or"star" (*) keys as well as a group of keys for menuselection: A, B, C and D. In the end, the letteredkeys were dropped from most phones, and it wasmany years before these keys became widely usedfor vertical service codes such as *67 in theUnited States and Canada to suppress caller ID.

Public payphones that accept credit cards usethese additional codes to send the informationfrom the magnetic strip.

The U.S. military also used the letters, relabeled, in their now defunct Autovonphone system[3]. Here they were used before dialing the phone in order to givesome calls priority, cutting in over existing calls if need be. The idea was to allowimportant traffic to get through every time. The levels of priority available wereFlash Override (A), Flash (B), Immediate (C), and Priority (D), with FlashOverride being the highest priority. Pressing one of these keys gave your callpriority, overriding other conversations on the network. Pressing C, Immediate,before dialing would make the switch first look for any free lines, and if all lineswere in use, it would disconnect any non-priority calls, and then any prioritycalls. Flash Override will kick every other call off the trunks between the originand destination. Consequently, it was limited to the White HouseCommunications Agency.

Precedence dialing is still done on the military phone networks, but usingnumber combinations (Example: Entering 93 before a number is a priority call)rather than the separate tones and the Government EmergencyTelecommunications Service has superseded Autovon for any civilian prioritytelco access.

Present-day uses of the A, B, C and D keys on telephone networks are few, andexclusive to network control. For example, the A key is used on some networks tocycle through different carriers at will (thereby listening in on calls). Their useis probably prohibited by most carriers. The A, B, C and D tones are used in


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1209 Hz on 697 Hz to makethe 1 tone

amateur radio phone patch and repeater operations to allow, among other uses,control of the repeater while connected to an active phone line.

DTMF tones are also used by some cable television networks and radio networksto signal the local cable company/network station to insert a local advertisementor station identification. These tones were often heard during a station IDpreceding a local ad insert. Previously, terrestrial television stations also usedDTMF tones to shut off and turn on remote transmitters.

DTMF signalling tones can also be heard at the start or end of some VHS (VideoHome System) cassette tapes. Information on the master version of the video tapeis encoded in the DTMF tone. The encoded tone provides information toautomatic duplication machines, such as format, duration and volume levels, inorder to replicate the original video as closely as possible.

DTMF tones are sometimes used in caller ID systems to transfer the caller IDinformation, however in the USA only Bell 202 modulated FSK signaling is usedto transfer the data.

A DTMF can be heard on most Whelen Outdoor Warning systems.

Keypad

Main article: Telephone keypad

The DTMF keypad is laid out in a 4×4 matrix, witheach row representing a low frequency, and eachcolumn representing a high frequency. Pressing asingle key (such as '1' ) will send a sinusoidal tonefor each of the two frequencies (697 and 1209hertz (Hz)). The original keypads had leversinside, so each button activated two contacts. Themultiple tones are the reason for calling thesystem multifrequency. These tones are thendecoded by the switching center to determinewhich key was pressed.


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DTMF keypad frequencies (with soundclips)

1209 Hz 1336 Hz 1477 Hz 1633 Hz

697 Hz 1 2 3 A

770 Hz 4 5 6 B

852 Hz 7 8 9 C

941 Hz * 0 # D

Special tone frequencies

National telephone systems define additional tones to indicate the status of lines,equipment, or the result of calls with special tones. Such tones are standardizedin each country and may consist of single or multiple frequencies. MostEuropean countries use a single frequency, where the United States uses a dualfrequency system, presented in the following table.

Event Low frequency High frequency

Busy signal 480 Hz 620 Hz

Ringback tone (US) 440 Hz 480 Hz

Dial tone 350 Hz 440 Hz

The tone frequencies, as defined by the Precise Tone Plan, are selected such thatharmonics and intermodulation products will not cause an unreliable signal. Nofrequency is a multiple of another, the difference between any two frequenciesdoes not equal any of the frequencies, and the sum of any two frequencies doesnot equal any of the frequencies. The frequencies were initially designed with aratio of 21/19, which is slightly less than a whole tone. The frequencies may notvary more than ±1.8% from their nominal frequency, or the switching center willignore the signal. The high frequencies may be the same volume or louder as thelow frequencies when sent across the line. The loudness difference between thehigh and low frequencies can be as large as 3 decibels (dB) and is referred to as"twist." The minimum duration of the tone should be at least 70 ms, although insome countries and applications DTMF receivers must be able to reliably detectDTMF tones as short as 45ms.

As with other multi-frequency receivers, DTMF was originally decoded by tunedfilter banks. Late in the 20th century most were replaced with digital signalprocessors. DTMF can be decoded using the Goertzel algorithm.


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See also

Goertzel algorithm (used for DTMF detection/decoding)Multi-frequencyPulse dialingPush-button telephoneRotary dialSelective calling (use of DTMF in two-way radio)Special information tone (e.g. "This number is not in service, and there is nonew number.")Telephone keypad

References

^ What is a Touch Tone Telephone? (http://www.wisegeek.com/what-is-a-touch-tone-telephone.htm) - Written by Malcolm Tatum - Last Modified: 22 January 2010

1.

^ United States Patent & Trademark Office, registered trademark serial number72109459, first used July 5, 1960

2.

^ "What are the ABCD tones?" - Tech FAQ (http://www.tech-faq.com/abcd.shtml)3.

Schenker, L (1960), "Pushbutton Calling with a Two-Group Voice-Frequency Code" (http://docs.google.com/Doc?docid=accnfh3pfb3_205hfvzk5rs) , The Bell system technical journal39 (1): 235–255, ISSN 0005-8580 (http://www.worldcat.org/issn/0005-8580) , http://docs.google.com/Doc?docid=accnfh3pfb3_205hfvzk5rs.

Further reading

ITU's recommendations for implementing DTMF services (PDF)(http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-F.902-199502-I!!PDF-E&type=items)Pushbutton Calling with a Two-Group Voice-Frequency Code - The Bellsystem technical journal (ISSN 0005-8580) Schenker yr:1960 vol:39 iss:1pg:235-255 (http://docs.google.com/Doc?docid=accnfh3pfb3_205hfvzk5rs)

External links

ITU-T Recommendation Q.23 - Technical features of push-button telephonesets (PDF document) (http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.23-198811-I!!PDF-E&type=items)

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dual-tone multi-frequency signaling (dtmf

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